It is known to conduct surveys of television audiences to determine the popularity of television programs. Various systems have been developed for automatically determining which station is being viewed to reduce the interaction between the person conducting the survey and the television viewer.
U.S. Pat. No. 2,903,508 (Hathaway) teaches a survey system wherein the horizontal or vertical synchronization signals are subjected to a cyclical phase shift to "tag" the broadcast signal. Then, a magnetic induction pickup receives the synchronization signals emanated by a television receiver and processes these signals to detect the cyclical phase shift. A second receiver is tuned to a known station, and the cyclical phase shift signals are compared to those of the television set being monitored. When the cyclical phase shifts match, it is concluded that the television set being monitored is tuned to the same station as that of the known television receiver.
U.S. Pat. No. 3,130,265 (Leonard) teaches a system for determining the channel to which a television receiver is tuned which also relies upon detection of the phase of synchronization (sync) pulses. In this system, transmitters are controlled so that the sync pulse of each transmitter is out of phase by a known amount with respect to the sync pulses of all other transmitters. This system requires that the conductor of the survey have control over the broadcast transmitters.
Systems such as those shown in U.S. Pat. Nos. 3,312,900 (Jaffe) and 4,577,220 (Laxton et al.) detect the frequency to which a local oscillator of a receiver is tuned to determine the channel beign viewed by a television user.
In a system shown in U.S. Pat. No. 3,806,805 (Wall), a television receiver imposes a load variation pattern on the main power supply line which is representative of the channel to which the receiver is tuned. An audience measuring system is responsive to variations in the main power supply line to identify the station.
U.S. Pat. No. 3,372,233 (Currey) teaches a monitoring system wherein the sync signal of a monitored receiver is combined with the sync signal of a receiver tuned to a known station. The phase relationship of these two sync signals indicates whether the monitored receiver is tuned to the known station.